EP4640883A1

EP4640883A1 - Cold-rolled steel sheet and manufacturing method therefor

Info

Publication number: EP4640883A1
Application number: EP23907603.7A
Authority: EP
Inventors: Sang-Hyun Kim; Young-Roc Im; Chang-Hyo Seo; Ki-Taek Jung
Original assignee: Posco Co Ltd
Current assignee: Posco Holdings Inc
Priority date: 2022-12-21
Filing date: 2023-12-14
Publication date: 2025-10-29
Also published as: KR20240098911A; MX2025006131A; EP4640883A4; WO2024136317A1; JP2025538042A; CN120380186A

Abstract

The present invention relates to a cold-rolled steel sheet and a manufacturing method therefor and, more specifically, to a cold-rolled steel sheet that is preferably applicable to collision energy absorption members such as a body-in-white (BIW) structural element, and a manufacturing method for the cold-rolled steel sheet. One aspect of the present invention is to provide a cold-rolled steel sheet with excellent strength, excellent elongation, and excellent hole expansion properties, and a manufacturing method therefor.

Description

Technical Field

The present disclosure relates to a cold-rolled steel sheet and a manufacturing method therefor and, more specifically, to a cold-rolled steel sheet that is preferably applicable to collision energy absorption members such as a body-in-white (BIW) structural element, and a manufacturing method for the cold-rolled steel sheet.

Background Art

Recently, in the automobile industry, advanced countries, led by Europe, are actively conducting research to reduce the weight of the vehicle body due to fuel efficiency regulations and performance improvements. In the case of the steel industry, in order to respond to the demand for weight reduction from automobile companies, efforts are made to increase strength and further reduce a thickness of the steel sheet in the same grade as compared to competing materials (Mg, Al, CFRP, etc.). In addition to weight reduction, due to strengthening CO₂ emission regulations and rapid changes to the era of electric vehicles, safety regulations for automobile passengers and pedestrians are strengthened, and thus, stability and high strength of vehicle body materials are also demanded. Specifically, the demand for high-strength steels of 980 to 1180MPa has increased. In the case of 980MPa grade steels, in order to be used as collision energy absorbing materials, not only should the elongation be high to form complex shapes, but also excellent hole expandability should be ensured so that fracture does not occur due to axial deformation.
These structural members are required to have high yield strength and hole expandability in order to facilitate absorption of impact energy. A representative manufacturing method for increasing yield strength is a method of utilizing water cooling during continuous annealing. A representative technology of this method is Patent Document 1. Patent Document 1 relates to manufacturing a steel having a martensite volume ratio of 80 to 97% and a balance of ferrite by continuously annealing a steel including C in the content of 0.18 to 0.3% and cooling the steel to room temperature with water, and then performing an overaging treatment at a temperature of 120 to 300°C for 1 to 15 minutes. Ultra-high strength steel may be manufactured by tempering a cold-rolled steel sheet after rapid cooling to room temperature after annealing in a dual phase zone or a single phase zone. In this case, a yield strength and hole expandability are excellent, but the shape quality of the coil deteriorates due to temperature deviation in width and length directions, and problems such as poor material quality and reduced workability may occur depending on the area during processing roll-forming components.
Additionally, as the strength of the steel sheet generally increases, the elongation thereof decreases, which causes problems such as reduced formability, and thus, an application thereof as a material for cold stamping is limited. In order to form a steel into complex shapes, the elongation should be high, and a representative method for increasing the elongation is a method of introducing retained austenite and utilizing a TRIP phenomenon, as in Patent Document 2. However, when a large amount of ferrite is introduced to secure additional elongation in addition to the retained austenite, as in Patent Document 2, the yield strength and hole expandability may be inferior.
Accordingly, in order to solve the above-described problems, it is necessary to develop an ultra-high strength steel sheet with a tensile strength of 980 MPa or more and excellent elongation and hole expandability.

[Prior Art Document]

(Patent Document 1) Japanese Patent Publication No. 1992-289120
(Patent Document 2) Japanese Patent Publication No. 2002-382250

Summary of Invention

Technical Problem

An aspect of the present disclosure is to provide a cold-rolled steel sheet and a manufacturing method therefor.
A preferred aspect of the present disclosure is to provide a cold-rolled steel sheet having excellent strength, excellent elongation and excellent hole expandability and a manufacturing method therefor.

Solution to Problem

According to an embodiment of the present disclosure, provided is a cold-rolled steel sheet, including: by wt%, carbon (C): 0.15 to 0.25%, manganese (Mn): 1.5 to 2.5%, silicon (Si): 1.0 to 2.0%, phosphorus (P): 0.1% or less (excluding 0%), sulfur (S): 0.03% or less (excluding 0%), aluminum (Al): 0.01 to 0.1%, molybdenum (Mo): 0.01% or less (excluding 0%), boron (B): 0.001% or less (excluding 0%), and a balance of Fe and other inevitable impurities, wherein a microstructure includes, in area%, ferrite: more than 10% and 45% or less, retained austenite: 7 to 15%, fresh martensite: 10% or less (including 0%), and a sum of tempered martensite and bainite: 40 to 80%, and the cold-rolled steel sheet includes a soft layer having a predetermined depth (t) of the steel sheet in a thickness direction from a surface thereof, and the soft layer satisfies the following relational expressions 1 and 2, $[C_{t / 5}] / [C_{M}] < 0.3$ $[C_{3 t} / 5] / [C_{M}] < 0.6$
(where, in the relational expression 1 and 2, [C_t/5] represents an average C content in a region from a surface to 1/5 of a predetermined depth (t) of the steel sheet in a thickness direction, [C_3t/5] represents an average C content in a region corresponding to 3/5 of the predetermined depth (t) of the steel sheet in the thickness direction from 1/5 of the predetermined depth (t), and [C_M] represents an average C content of the steel sheet).
A fraction of the tempered martensite may be 40% or more.
The predetermined depth (t) may be 50 to 100µm.
The cold-rolled steel sheet may have one of a hot-dip galvanized layer (GI), an alloyed hot-dip galvanized layer (GA), and an electrolytic galvanized layer (EG) formed on at least one surface thereof. The cold-rolled steel sheet may have a 30000MPa% ≤ X = yield strength × [total elongation + (2 × hole expansion ratio)] ≤ 70000MPa%.
According to another embodiment of the present disclosure, provided is a method for manufacturing a cold-rolled steel sheet, including: heating a slab at 1100 to 1300°C, the slab including, by wt%, carbon (C): 0.15 to 0.25%, manganese (Mn): 1.5 to 2.5%, silicon (Si): 1.0 to 2.0%, phosphorus (P): 0.1% or less (excluding 0%), sulfur (S): 0.03% or less (excluding 0%), aluminum (Al): 0.01 to 0.1%, molybdenum (Mo): 0.01% or less (excluding 0%), boron (B): 0.001% or less (excluding 0%), and a balance of Fe and other inevitable impurities; finishing hot-rolling the heated slab at Ar3 or higher to obtain a hot-rolled steel sheet; coiling the hot-rolled steel sheet at 700°C or less; pickling and then cold-rolling the coiled hot-rolled steel sheet to obtain a cold-rolled steel sheet; primarily heating the cold-rolled steel sheet at 780°C or higher and less than Ac3-10°C for 30 seconds or longer under an atmosphere condition having a dew point temperature of 0 to 30°C; primarily cooling the heated cold-rolled steel sheet to 600 to 750°C at an average cooling rate of 1 to 10°C/s; secondarily cooling the primarily-cooled cold-rolled steel sheet to 150°C~Ms at an average cooling rate of 10 to 45°C/s; and secondarily heating the secondarily-cooled cold-rolled steel sheet at Ms~480°C and then subjecting the steel sheet to an overaging treatment for 1 to 30 minutes.
The cold rolling may be performed at a cold reduction ratio of 30 to 80%.
The method for manufacturing a cold-rolled steel sheet may further include: immersing the cold-rolled steel sheet in a hot-dip galvanized bath at 440 to 480°C after the secondary heating and holding, to form a hot-dip galvanized layer.
The method for manufacturing a cold-rolled steel sheet may further include: performing an alloying heat treatment on the cold-rolled steel sheet at 450 to 520°C after the formation of the hot-dip galvanized layer.
The method for manufacturing a cold-rolled steel sheet may further include: forming an electro-galvanized layer after the secondary heating and holding.

Advantageous Effects of Invention

According to an aspect of the present disclosure, a cold-rolled steel sheet and a manufacturing method therefor could be provided.
According to a preferred aspect of the present disclosure, a cold-rolled steel sheet having excellent strength, excellent elongation and excellent hole expandability and a manufacturing method therefor could be provided.

Brief Description of Drawings

FIG. 1 is a microstructure image of Inventive Example 1 according to an embodiment of the present disclosure observed with an SEM microscope.

Best Mode for Invention

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the present disclosure. The singular forms used herein include the plural forms as well, unless the context clearly dictates otherwise. The meaning of "include" and "comprise" used in the specification specifies particular features, regions, integers, steps, operations, elements, and/or components, but does not exclude the presence or addition of other particular features, regions, integers, steps, operations, elements, components, and/or groups.
Unless otherwise defined, all terms including technical terms and scientific terms used herein have the same meaning as generally understood by one of ordinary skill in the art to which example embodiments of the present disclosure belong. Terms defined in commonly used dictionaries are additionally interpreted as having a meaning consistent with the relevant technical literature and the presently disclosed content, and are not interpreted in an idealized or overly formal sense unless expressly so defined herein.
Hereinafter, a cold-rolled steel sheet according to an embodiment of the present disclosure will be described. First, an alloy composition will be described. The content of the alloy composition described below refers to wt% unless otherwise specified.

Carbon (C): 0.15 to 0.25%

C is an interstitial solid-solution element and is the most effective and important element for improving the strength of steel. When the content of C is less than 0.15%, it may be difficult to obtain the yield ratio and tensile strength targeted in the present disclosure. When the content of C exceeds 0.25%, the strength thereof may increase rapidly due to excessive formation of martensite during cooling due to an increase in hardenability, which may result in poor elongation and reduced weldability. Accordingly, it is preferable that the content of C is in the range of 0.15 to 0.25%. A lower limit of the content of C is more preferably 0.18%, and a lower limit of the content of C is more preferably 0.2%. The upper limit of the content of C is more preferably 0.24%.

Manganese (Mn): 1.5 to 2.5%

Mn is an element added to secure strength. When the content of Mn is less than 1.5%, it may be difficult to secure the level of strength desired in the present disclosure. When the content of Mn exceeds 2.5%, an Ms temperature decreases during cooling after annealing, which may make it difficult to smoothly secure an initial martensite phase. It may be difficult to simultaneously secure the strength, elongation, and hole expandability targeted in the present disclosure due to a decrease in a tempered martensite fraction in a Quenching & Partitioning (Q&P) process. Additionally, Mn may segregate in a thickness direction, making it easy to form an Mn band in a slab, which may increase the possibility of defects occurring during a rolling process along with continuous casting cracks. Accordingly, the content of Mn is preferably in the range of 1.5 to 2.5%. A lower limit of the content of Mn is more preferably 1.8%, and a lower limit of the content of Mn is more preferably 2.0%. The upper limit of the content of Mn is more preferably 2.4%.

Silicon (Si): 1.0 to 2.0%

Si is a key element of Transformation Induced Plasticity (TRIP) steel securing an appropriate level of retained austenite fraction and increasing an elongation by suppressing precipitation of cementite. When the content of Si is less than 1.0%, the control of cementite precipitation during the reheating and overaging operations may be not smoothly performed, and thus, a fraction of a finally obtained retained austenite may be reduced or the stability thereof may be low, resulting in a poor elongation. On the other hand, when the content of Si exceeds 2.0%, the properties of a welded portion may deteriorate due to the occurrence of Liquid Metal Embrittlement (LME) cracks, and the surface characteristics and plating properties of a steel may also deteriorate. Accordingly, the content of Si is preferably in the range of 1.0 to 2.0%. A lower limit of the content of Si is more preferably 1.2%. An upper limit of the content of Si is more preferably 1.8%.

Phosphorus (P): 0.1% or less (excluding 0%)

P is an impurity element included in steel, and when a content thereof exceeds 0.1%, weldability thereof deteriorates and there is a risk of steel brittleness. The lower the content of P is, the more advantageous it is, but considering that phosphorus (P) is inevitably included in the manufacturing process, 0% is excluded. Accordingly, the content of P is preferably in the range of 0.1% or less (excluding 0%). The content of P is more preferably 0.03% or less.

Sulfur (S): 0.03% or less (excluding 0%)

S, similarly to P, is an impurity element included in steel, and when a content thereof exceeds 0.03%, ductility and weldability thereof may deteriorate. The lower the S content is, the more advantageous it is, but considering that sulfur (S) is inevitably included in the manufacturing process, 0% is excluded. Accordingly, the content of S is preferably in the range of 0.03% or less (excluding 0%). The content of S is more preferably 0.005% or less.

Aluminum (Al): 0.01 to 0.1%

Al is an element added to remove oxygen in molten steel, and is effective in stabilizing retained austenite by suppressing precipitation of cementite during reheating and overaging operations, similarly to Si. When the content of Al is less than 0.01%, the deoxidation effect may not be sufficiently obtained, which may impair the cleanliness of a steel. When the content of Al exceeds 0.1%, not only may the castability of the slab deteriorate, but the temperature required for single-phase region heating during annealing may also increase, which may cause production and facility problems. Accordingly, the content of Al is preferably in the range of 0.01 to 0.1%. An upper limit of the content of Al is more preferably 0.05%.

Molybdenum (Mo): 0.01% or less (excluding 0%)

Mo is a representative element that may improve hardenability, but in the present disclosure, since the balance of strength, an elongation and hole expandability is important, and a steel has a tensile strength of 980 MPa, there is no need to add Mo for the purpose of improving hardenability and facilitating the formation of martensite. When Mo is added, there is a problem of increasing the manufacturing costs, so that it is preferable not to intentionally add Mo in the present disclosure. Accordingly, in the present disclosure, the content of Mo may be limited to 0.01% or less. Meanwhile, considering a case in which Mo is inevitably included during the manufacturing process, a lower limit thereof may be 0.001%.

Boron (B): 0.001% or less (excluding 0%)

B is a representative element that may improve hardenability, but in the present disclosure, since the balance of strength, an elongation and hole expandability is important, and a steel has a tensile strength of 980 MPa, there is no need to add B for the purpose of improving hardenability and facilitating the formation of martensite. Accordingly, it is preferable not to add B in the present disclosure. Accordingly, in the present disclosure, the content of B may be limited to 0.001% or less. Meanwhile, considering a case in which B is unavoidably included during the manufacturing process, a lower limit thereof may be 0.0001%.
The remaining component is iron (Fe). However, since unintended impurities may inevitably be mixed during a normal manufacturing process from raw materials or the surrounding environment, this may not be excluded. Since these impurities may be known to anyone who is skilled in the normal manufacturing process, not all of their contents are specifically mentioned in this specification.
A microstructure of the cold-rolled steel sheet of the present disclosure may include, in area%, ferrite: more than 10% and 45% or less, retained austenite: 7 to 15%, fresh martensite: 10% or less (including 0%), and a sum of tempered martensite and bainite: 40 to 80%. The ferrite is a structure advantageous for securing an elongation. When a fraction of the ferrite is 10% or less, it may be difficult to secure the elongation targeted in the present disclosure, and when the fraction of the ferrite exceeds 45%, it may be difficult to secure the strength and hole expandability targeted in the present disclosure. The retained austenite is a structure absolutely necessary for securing the elongation together with the ferrite formed during the annealing process. When the fraction of the retained austenite is less than 7%, it may be difficult to secure the elongation targeted in the present disclosure. When the fraction of the retained austenite exceeds 15%, it may be difficult to secure the targeted elongation due to insufficient stability of the retained austenite. When the fraction of the fresh martensite exceeds 10%, it may be difficult to obtain a steel having excellent strength, an excellent elongation and excellent hole expandability characteristics. The tempered martensite and bainite are structures necessary for securing strength and hole expandability, and when a phase transformation is advanced to include the fraction in the above-described range, the retained austenite that is stable at room temperature may ultimately include 7 to 15%. When the fraction of the tempered martensite and bainite is less than 40%, it may be difficult to secure the retained austenite fraction targeted by the present disclosure due to insufficient total transformation amount, and since 10% or more of the fresh martensite is secured, it may be difficult to secure the strength, elongation, and hole expandability targeted by the present disclosure. When the fraction of the tempered martensite and bainite exceeds 80%, it may be possible to secure strength and hole expandability, but it may be difficult to secure the high elongation targeted by the present disclosure due to the lack of the fraction of ferrite and retained austenite. Meanwhile, the fraction of the tempered martensite is more preferably 40% or more.
The cold-rolled steel sheet of the present disclosure includes a soft layer having a predetermined depth (t) of the steel sheet in a thickness direction from a surface thereof, and it is preferable that the soft layer satisfies the following relational expressions 1 and 2. By forming the soft layer, Liquid Metal Embrittlement (LME) cracks may be prevented. The predetermined depth (t) may be 50 to 100µm.More specifically, the predetermined depth (t) may be 50 to 80µm. In this case, the surface refers to a surface of a base steel sheet, and a plating layer that may be formed on the surface of the base steel sheet is excluded.
It is preferable that the soft layer satisfies the following relational expressions 1 and 2. $[C_{t / 5}] / [C_{M}] < 0.3$ $[{C 3}_{t / 5}] / [C_{M}] < 0.6$
(where, in the relational expressions 1 and 2, [C_t/5] represents an average C content in a region from the surface to 1/5 of the predetermined depth (t) of the steel sheet in a thickness direction, [C_3t/5] represents an average C content in a region corresponding to 3/5 of the predetermined depth (t) of the steel sheet in the thickness direction from 1/5 of the predetermined depth (t), and [C_M] represents an average C content of the steel sheet.)
When the conditions of the relational expressions 1 and 2 are not satisfied, it may be difficult to obtain good LME characteristics.
Meanwhile, the [C_1/5t] may include a microstructure including, area%, 80% or more of ferrite, and a balance of bainite and tempered martensite.
The cold-rolled steel sheet of the present disclosure may have one of a hot-dip galvanized layer (GI), an alloyed hot-dip galvanized layer (GA), and an electrogalvanized layer (EG) formed on at least one surface thereof. The present disclosure does not specifically limit the specific conditions of the hot-dip galvanized layer (GI), the alloyed hot-dip galvanized layer (GA), or the electrogalvanized layer (EG), and all types commonly used in the relevant technical field may be used.
The cold-rolled steel sheet of the present disclosure may have a yield strength (YS): 600 MPa or more, a tensile strength (TS): 980 MPa or more, a total elongation (T-El): 21% or more, and a hole expansion ratio (HER): 20 to 40%. Since the yield strength, the tensile strength, the total elongation, and the uniform elongation are advantageous as they are higher, the present disclosure does not specifically limit upper limits of the yield strength, the tensile strength, and the total elongation.
The cold-rolled steel sheet of the present disclosure may be 30000 MPa% ≤ X = yield strength × [total elongation + (2 × hole expansion ratio)] ≤ 70000 MPa%. The control of the value X is to secure the yield strength of 600 MPa or more, targeted by the present disclosure, while simultaneously securing excellent elongation and excellent hole expandability. When the value of X is less than 30000 MPa% or exceeds 70000 MPa%, one or more of the properties of the strength, the elongation, and the hole expandability, desired by the present disclosure, may be inferior, making it difficult to use the cold-rolled steel sheet as a member for absorbing impact energy. A lower limit of the value X is more preferably 35000 MPa%. An upper limit of the value X is more preferably 65000 MPa%, and 60000 MPa% is even more preferably.
Hereinafter, a method for manufacturing a cold-rolled steel sheet according to an embodiment of the present disclosure will be described.
First, the slab is heated to a temperature of 1100 to 1300°C. The slab heating is performed to smoothly perform the subsequent hot rolling process and obtain the target properties of the steel sheet. When the slab heating temperature is lower than 1100°C, a problem of a rapid increase in the hot rolling load may occur. When the slab heating temperature exceeds 1300°C, the amount of surface scale may increase, which may reduce productivity.
Then, the heated slab is subjected to a finishing hot rolling at Ar3 or higher to obtain a hot rolled steel sheet. When the finishing hot rolling temperature is lower than Ar3, a two-phase zone of ferrite + austenite or a ferrite zone rolling is performed, resulting in generating a mixed grain structure, and equipment malfunction may occur due to a change in the hot rolling load. Meanwhile, the Ar3 may be obtained through the following relational expression 1. [Relational Expression 1] Ar3(°C) = 910 - 203√[C] + 44.7[Si] + 31.5[Mo] - 30[Mn] - 11[Cr] + 700[P] + 400[Al] + 400[Ti]
Then, the hot-rolled steel sheet is coiled at 700°C or lower. When a coiling temperature exceeds 700°C, an oxide film may be excessively formed on the surface of the steel sheet, which may cause defects. The coiling temperature is more preferably 650°C or lower. On the other hand, as the coiling temperature decreases, the strength of the hot-rolled steel sheet increases, which has the disadvantage of increasing the rolling load as the subsequent cold rolling process, but since this is not a factor making actual production impossible, the present disclosure does not specifically limit a lower limit thereof. However, as an example, the lower limit of the coiling temperature may be 300°C.
Then, the coiled hot-rolled steel sheet is pickled and then cold-rolled to obtain a cold-rolled steel sheet. The pickling is a process for removing the oxide layer formed on the surface of the coiled hot-rolled steel sheet. The cold-rolling may be performed at a cold reduction ratio of 30 to 80%. When the cold reduction ratio is less than 30%, it is difficult to secure the target thickness, and there is a concern that the formation of austenite and securing of physical properties may be affected during annealing heat treatment due to retained crystal grains formed during hot rolling. When the cold reduction ratio exceeds 80%, a material deviation may occur due to the uneven rolling amount in length and width directions due to the work hardening occurring during cold rolling, and it may be difficult to secure a target thickness due to the rolling load.
Then, the cold-rolled steel sheet is primarily heated to a temperature of 780°C or higher and less than Ac3-10°C for 30 seconds or longer under atmospheric conditions having a dew point temperature of 0 to 30°C. The primary heating is to form some annealed ferrite in addition to the retained austenite in order to secure an elongation of 21% or more. When the dew point temperature is less than 0°C, the soft layer targeted by the present disclosure is not sufficiently formed on the surface of the steel sheet. When the dew point temperature exceeds 30°C, there are problems of reduced equipment life and productivity. A lower limit of the dew point temperature is more preferably 2°C. A upper limit of the dew point temperature is more preferably 25°C. When the primary heating temperature is less than 780°C, annealed ferrite may be excessively formed, making it difficult to secure strength and hole expandability. When the primary heating temperature is Ac3-10°C or higher, the fraction of annealed ferrite may be insufficient due to heating at a single-phase region level, resulting in poor elongation. A lower limit of the primary heating temperature is more preferably 790°C. An upper limit of the primary heating temperature is more preferably Ac3-15°C. When the primary heating time is less than 30 seconds, there is a disadvantage in that a sufficient annealing effect is not obtained. Meanwhile, the longer the primary heating time is, the more advantageous it is, and thus the present disclosure is not particularly limited to the lower limit thereof. However, as an example, the upper limit of the primary heating time may be 500 seconds. Meanwhile, the above-described Ac3 may be obtained through the following relational expression 2. Ac3(°C) = 910 - 203√[C] - 15.2[Ni] + 44.7[Si] + 104[V] + 31.5[Mo] + 13.1[W]
Then, the heated cold-rolled steel sheet is primarily cooled to 600 to 750°C at an average cooling rate of 1 to 10°C /s. When a primary cooling end temperature is less than 600°C, there is a concern that phases such as ferrite or bainite may be formed, resulting in a decrease in strength. When the primary cooling end temperature exceeds 750°C, problems may occur in an actual production line. A lower limit of the primary cooling end temperature is more preferably 610°C, and more preferably 630°C. An upper limit of the primary cooling end temperature is more preferably 740°C, and more preferably 730°C. When a primary average cooling rate is less than 1°C/s, it may be difficult to secure the target strength because ferrite is formed during cooling. When the primary average cooling rate exceeds 10°C/s, the average cooling rate decreases during the second cooling, making it difficult to secure sufficient martensite, which in turn leads to a decrease in the fraction of tempered martensite, making it difficult to secure strength and hole expandability at the same time. An upper limit of the primary average cooling rate is more preferably 6°C/s.
Then, the primarily-cooled cold-rolled steel sheet is secondarily cooled to 150°C~Ms at an average cooling rate of 10 to 45°C/s. In order to secure a tempered martensite structure of 40% or more required in the present disclosure, it is necessary to cool the steel sheet between the martensite transformation start and finish temperature (Martensite Start (Ms) to Finish Temperature (Mf)) during the secondary cooling. That is, for this purpose, the secondary cooling end temperature preferably has a range of 150°C-Ms. When the secondary cooling end temperature is less than 150°C, a tempered martensite fraction may become excessively high, the retained austenite fraction may decrease, and the elongation may become inferior. When the secondary cooling end temperature exceeds Ms, the formation of tempered martensite structure may become difficult, and the strength and hole expandability may become inferior. A lower limit of the secondary cooling end temperature is more preferably 180. When the secondary average cooling rate is less than 10°C/s, some bainite structure may be formed from the primary cooling section to the secondary cooling. When the secondary average cooling rate exceeds 45°C/s, a surface shape of the steel sheet may become inferior due to the rapid martensite transformation rate at the time of the secondary cooling, and a material deviation problem in a width direction may occur. A lower limit of the secondary average cooling rate is more preferably 12°C/s. An upper limit of the secondary average cooling rate is more preferably 42°C/s. Meanwhile, the above-described Ms may be obtained through the following relational expression 3. Ms (°C) = 539 - 423 [C] - 30.4 [Mn] - 7.5 [Si] + 30 [Al] - 12.1 [Cr] - 17.7 [Ni] - 7.5 [Mo]
Then, the secondarily-cooled cold-rolled steel sheet is secondarily heated to a temperature of Ms~480°C, and then subjecting to overaging for 1 to 30 minutes. The secondary heating and overaging are intended to improve the toughness by changing the high dislocation density and hard martensite formed during the second cooling into tempered martensite. In addition, by securing a sufficient amount of tempered martensite and bainite during the secondary heating and overaging treatment, C is enriched in the austenite remaining from the annealing (Partitioning). In this process, the martensite transformation start temperature (Ms) of the austenite in which C is enriched is lowered to a temperature equal to or lower than room temperature, and a large amount of retained austenite is ultimately formed, thereby securing the properties targeted by the present disclosure. When the secondary heating temperature is less than Ms or exceeds 480°C, it may be difficult to secure the fraction of the microstructure targeted by the present disclosure. A lower limit of the secondary heating temperature is more preferably 360°C. An upper limit of the secondary heating temperature is more preferably 460°C. When the overaging treatment time is less than 1 minute, it is difficult to obtain a partitioning effect because sufficient transformation is not advanced. When the overaging treatment time exceeds 30 minutes, the secondary heating and overaging treatment section should be significantly long, and productivity decreases, so that it may be difficult to apply to an actual production line.
Meanwhile, after the secondary heating and maintenance, the cold-rolled steel sheet may be immersed in a hot-dip galvanized bath of 440 to 480°C to form a hot-dip galvanized layer. When a hot-dip galvanized bath temperature is less than 440°C, it may be difficult to manage the molten zinc plating bath, and when the hot-dip galvanized bath temperature exceeds 480°C, a final elongation may decrease.
Additionally, after the formation of the hot-dip galvanized layer, the cold-rolled steel sheet on which the hot-dip galvanized layer is formed may be subjected to an alloying heat treatment at 450 to 520°C. When the alloying heat treatment temperature is less than 450°C, it may be difficult to form a sufficient Fe-Zn alloy plating layer, and when the alloying heat treatment temperature exceeds 520°C, the final elongation may be inferior due to the decomposition of the retained austenite formed in a previous operation.
On the other hand, after the secondary heating and maintenance, an electro-galvanized layer may be formed.

Mode for Invention

Hereinafter, the present disclosure will be described in more detail through embodiments. However, it should be noted that the following embodiments are intended only to explain the present disclosure through examples, and are not intended to limit the scope of the present disclosure. This is because the scope of the present disclosure is determined by the matters described in the claims and matters reasonably inferred therefrom.

(Example)

A slab having an alloy composition as described in Table 1 below was heated to a temperature of 1100 to 1300°C, and then finishing hot-rolled at 900 to 1000°C to manufacture a hot-rolled steel sheet. The hot-rolled steel sheet was coiled at 350 to 650°C, pickled, and cold-rolled at a cold reduction ratio of 45 to 65%, and then, a cold-rolled steel sheet was manufactured by applying the conditions described in Table 2 below. Meanwhile, the conditions described in Table 2 below are based on a surface temperature of the steel sheet. Accordingly, the manufactured cold-rolled steel sheet was subjected to hot-dip galvanizing or hot-dip galvanizing and alloying heat treatment under the conditions described in Table 2 below.
The microstructure and mechanical properties of the cold-rolled steel sheet manufactured in this manner were measured, and the results thereof are described in Tables 3 and 4 below.
A phase fraction of the microstructure was measured using XRD and EBSD for t/4 (t: thickness of steel) of the cold-rolled steel sheet.
The formation of a soft layer having a predetermined depth (t) was measured using GDS. An average C content [C_t/5] of a region from a surface thereof to 1/5 of a predetermined depth (t) of the steel sheet in a thickness direction and an average C content [C_3t/5] of a region corresponding to 3/5 of the predetermined depth (t) of the steel sheet in the thickness direction from 1/5 of the predetermined depth (t) were calculated by the average value of the content of C measured through GDS, and [C_M] was measured using the results of OES and ICP C component analysis of a parent material.
Yield strength (YS), tensile strength (TS), total elongation (T-El), and uniform elongation (U-El) were measured by processing the cold-rolled steel sheet into specimens of the JIS standard (gauge length width × length: 25 × 50 mm, total length of specimen: 200 to 260 mm), and then performing a tensile test at a test speed of 28 mm/min.
Hole expansion ratio (HER) was measured according to the ISO 16330 standard, and holes were sheared at a clearance of 12% using a 10 mm diameter punch.

LME was evaluated according to the ISO 18278-2 standard, and ∘ was indicated when a C-type crack occurred, and × was indicated when no cracking occurred.

Table 1:

Steel Type No.	Alloy Composition (wt%)
Steel Type No.	C	Mn	Si	P	S	Al	Mo	B
Comparative Steel 1	0.201	2.36	1.37	0.0071	0.0012	0.036	0.1	0.0015
Comparative Steel 2	0.205	1.27	1.47	0.0082	0.0011	0.035	0.002	0.0003
Inventive Steel 1	0.215	2.3	1.45	0.0065	0.0004	0.031	0.003	0.0004
Inventive Steel 2	0.221	2.25	1.5	0.0055	0.0005	0.033	0.002	0.0003
Inventive Steel 3	0.224	2.26	1.48	0.0058	0.0006	0.025	0.002	0.0004

Table 2:

Division	Steel Type No.	Primary Heating			Primary Cooling		Ms (°C )	Secondary Cooling		Sec ond ary Hea tin g Tem per atu re (°C)	Over agin g Trea tmen t Time (min )	Hot-dip galva nized Bath Tempe ratur e (°C)	Tempera ture of Alloyin g Heat Treatme nt (°C)
Division	Steel Type No.	Dew poin t temp erat ure (°C)	Tem per atu re (°C)	Tim e (se con ds)	End Temp erat ure (°C)	Avera ge Cooli ng Rate (°C/s )	Ms (°C )	End Tempe ratur e (°C)	Avera ge Cooli ng Rate (°C/s )	Sec ond ary Hea tin g Tem per atu re (°C)	Over agin g Trea tmen t Time (min )	Hot-dip galva nized Bath Tempe ratur e (°C)	Tempera ture of Alloyin g Heat Treatme nt (°C)
Inventive Example 1	Inventive Steel 1	12	840	100	700	3.09	356	300	20.38	400	166	460	-
Inventive Example 2	Inventive Steel 1	15	840	87	680	4.03	356	310	21.54	420	145	460	-
Inventive Example 3	Inventive Steel 2	14	820	87	650	4.28	354	200	26.20	400	145	460	-
Inventive Example 4	Inventive Steel 3	16	830	87	700	3.28	352	250	26.20	400	145	460	-
Inventive Example 5	Inventive Steel 1	13	850	70	680	2.28	356	300	13.52	400	206	460	480
Inventive Example 6	Inventive Steel 2	11	840	53	700	2.50	354	250	21.35	400	155	460	480
Inventive Example 7	Inventive Steel 3	12	850	53	700	2.68	352	320	18.03	400	155	460	500
Inventive Example 8	Inventive Steel 2	14	840	100	700	3.09	354	280	20.38	400	166	-	-
Inventive Example 9	Inventive Steel 2	15	830	87	680	3.78	354	260	24.45	400	145	-	-
Comparative Example 1	Inventive Steel 1	-50	840	100	700	3.09	356	300	20.38	400	166	460	-
Comparative Example 2	Inventive Steel 1	-48	840	87	680	4.03	356	310	21.54	420	145	460	-
Comparative Example 3	Inventive Steel 2	-50	820	87	650	4.28	354	200	26.20	400	145	460	-
Comparative Example 4	Inventive Steel 3	-47	830	87	700	3.28	352	250	26.20	400	145	460	-
Comparative Example 5	Inventive Steel 1	-1	840	87	680	4.03	356	310	21.54	420	145	460	-
Comparative Example 6	Inventive Steel 2	15	775	87	700	1. 89	354	250	26.20	400	145	460	-
Comparative Example 7	Inventive Steel 2	14	900	87	680	5.55	354	310	21.54	400	145	460	-
Comparative Example 8	Inventive Steel 2	12	850	87	590	6.55	354	200	22.71	400	145	460	-
Comparative Example 9	Inventive Steel 2	16	820	87	590	5.80	354	200	22.71	400	145	460	-
Comparative Example 10	Inventive Steel 2	12	840	87	700	3.53	354	400	17.47	400	145	460	-
Comparative Example 11	Inventive Steel 2	14	820	87	700	3.02	354	400	17.47	400	145	460	-
Comparative Example 12	Inventive Steel 2	11	850	87	680	4.28	354	300	22.13	500	145	460	-
Comparative Example 13	Inventive Steel 2	13	900	53	700	3.57	354	300	18. 98	400	155	460	480
Comparative Example 14	Inventive Steel 2	15	850	53	590	4.46	354	300	14.23	400	155	460	480
Comparative Example 15	Inventive Steel 2	16	840	53	700	2.50	354	400	14.23	400	155	460	480
Comparative Example 16	Inventive Steel 2	14	850	23	680	6.83	354	300	40.57	400	69	460	480
Comparative Example 17	Comparati ve Steel 1	15	840	87	680	4.03	362	300	22.13	400	145	460	480
Comparative Example 18	Comparati ve Steel 2	15	820	87	650	4.28	391	300	20.38	400	145	460	500
Ac3 (°C) = 910-203√[C]-15.2[Ni]+44.7[Si]+104[V]+31.5[Mo]+13.1[W]
Ms(°C) = 539-423[C]-30.4[Mn]-7.5[Si]+30[Al]-12.1[Cr]-17.7[Ni]-7.5[Mo]

Table 3:

Division	Microstructure (area%)					Relational Expression 1	Relational Expression 2
Division	F	RA	FM	B	TM	Relational Expression 1	Relational Expression 2
Inventive Example 1	16	9	4	11	60	○	○
Inventive Example 2	16	9	6	14	55	○	○
Inventive Example 3	30	7	5	12	46	○	○
Inventive Example 4	19	9	4	6	62	○	○
Inventive Example 5	15	9	3	8	65	○	○
Inventive Example 6	18	10	4	5	63	○	○
Inventive Example 7	15	8	5	12	60	○	○
Inventive Example 8	14	12	4	16	54	○	○
Inventive Example 9	22	11	4	10	53	○	○
Comparative Example 1	16	9	4	11	60	×	×
Comparative Example 2	16	9	6	14	55	×	×
Comparative Example 3	30	7	5	12	46	×	×
Comparative Example 4	19	9	4	6	62	×	×
Comparative Example 5	16	9	6	14	55	×	○
Comparative Example 6	52	5	12	31	0	○	○
Comparative Example 7	2	8	9	7	74	○	○
Comparative Example 8	19	6	7	3	65	○	○
Comparative Example 9	39	5	11	20	35	○	○
Comparative Example 10	16	9	10	65	0	○	○
Comparative Example 11	25	8	12	55	0	○	○
Comparative Example 12	14	6	14	12	54	○	○
Comparative Example 13	0	8	6	8	78	○	○
Comparative Example 14	19	8	9	19	45	○	○
Comparative Example 15	16	8	10	66	0	○	○
Comparative Example 16	11	5	15	3	66	○	○
Comparative Example 17	2	7	7	3	81	○	○
Comparative Example 18	58	8	5	10	19	○	○
F: Ferrite, RA: Remaining austenite, FM: Fresh Martensite, B: Bainite, and TM: Tempered Martensite
[Relational Expression 1] [C_t/5]/[C_M] < 0.3
[Relational Expression 2] [C_3t/5] / [C_M] < 0.6
(where, in the relational expressions 1 and 2, [C_t/5] represents an average C content in a region from a surface to 1/5 of a predetermined depth (t) of the steel sheet in a thickness direction, [C_3t/5] represents an average C content in a region corresponding to 3/5 of the predetermined depth (t) of the steel sheet in the thickness direction from 1/5 of the predetermined depth (t), and [C_M] represents an average C content of the steel sheet,)

Table 4:

Division	Yield Strength (MPa)	Tensile Strength (MPa)	Total Elongati on (%)	Uniform Elongat ion (%)	Hole Expandabi lity (%)	X	LME Crack (C Type)
Inventive Example 1	622	1033	22	15	32	53492	×
Inventive Example 2	605	1031	22	14	27	45980	×
Inventive Example 3	618	1005	22	15	25	44496	×
Inventive Example 4	643	1042	22	15	24	45010	×
Inventive Example 5	691	1035	22	15	26	51134	×
Inventive Example 6	672	1021	22	15	25	48384	×
Inventive Example 7	644	1019	21	15	24	44436	×
Inventive Example 8	705	1027	21	15	23	47235	×
Inventive Example 9	683	1015	23	15	28	53957	×
Comparative Example 1	624	1083	22	15	33	54912	○
Comparative Example 2	602	1081	21	14	26	43946	○
Comparative Example 3	614	1063	22	15	26	45436	○
Comparative Example 4	649	1071	22	15	25	46728	○
Comparative Example 5	613	1049	22	14	27	46588	○
Comparative Example 6	521	1065	20	15	16	27092	×
Comparative Example 7	752	1077	19	13	50	89488	×
Comparative Example 8	728	1069	19	13	45	79352	×
Comparative Example 9	531	1058	22	15	14	26550	×
Comparative Example 10	506	1045	19	14	15	24794	×
Comparative Example 11	487	1057	20	15	12	21428	×
Comparative Example 12	577	1093	15	10	18	29427	×
Comparative Example 13	671	1052	19	14	48	77165	×
Comparative Example 14	488	1050	19	14	13	21960	×
Comparative Example 15	491	1059	20	15	12	21604	×
Comparative Example 16	555	1070	16	11	10	19980	×
Comparative Example 17	851	1105	15	10	38	77441	×
Comparative Example 18	425	912	25	18	20	27625	×
X = Yield strength × [Total elongation + (2×Hole Expandability)]

As shown in Tables 1 to 4, in the case of Inventive Examples 1 to 9 satisfying the alloy composition and manufacturing conditions proposed by the present disclosure, it can be seen that excellent physical properties are secured by securing the microstructure and the soft layer targeted by the present disclosure.
In the case of Comparative Examples 1 to 4, it can be seen that the strength, elongation, and hole expandability required in the present disclosure are satisfied, but the LME characteristics are inferior because the soft layer is not secured as the dew point temperature is not satisfied, among the manufacturing conditions.
For Comparative Example 5, it can be seen that the strength, elongation, and hole expandability required by the present disclosure are satisfied, but among the manufacturing conditions, decarburization is weak as the dew point temperature is not satisfied, and thus, the LME characteristics are inferior because the relational expression 1 is not satisfied.
In the case of Comparative Examples 6 to 16, it can be seen that as the manufacturing conditions of the present disclosure are not satisfied, the microstructure targeted by the present disclosure is not ensured, and thus, one or more of the strengths, the elongation and the hole expandability required by the present disclosure are not satisfied.
In the case of Comparative Examples 17 and 18, it can be seen that the microstructure targeted by the present disclosure is not ensured because the alloy composition or the alloy composition and manufacturing conditions of the present disclosure are not satisfied, and thus, one or more of the strength, the elongation, and the hole expandability required by the present disclosure are not satisfied.
FIG. 1 is a microstructure image of Inventive Example 1 observed with an SEM microscope. As can be seen from FIG. 1, in the case of Inventive Example 1, it can be seen that the microstructure of an appropriate fraction to be obtained by the present disclosure is secured.

Claims

A cold-rolled steel sheet, comprising: by wt%, carbon (C): 0.15 to 0.25%, manganese (Mn): 1.5 to 2.5%, silicon (Si): 1.0 to 2.0%, phosphorus (P): 0.1% or less (excluding 0%), sulfur (S): 0.03% or less (excluding 0%), aluminum (Al): 0.01 to 0.1%, molybdenum (Mo): 0.01% or less (excluding 0%), boron (B): 0.001% or less (excluding 0%), and a balance of Fe and other inevitable impurities,
wherein a microstructure includes, in area%, ferrite: more than 10% and 45% or less, retained austenite: 7 to 15%, fresh martensite: 10% or less (including 0%), and a sum of tempered martensite and bainite: 40 to 80%,

the cold-rolled steel sheet includes a soft layer having a predetermined depth (t) of the steel sheet in a thickness direction from a surface thereof, and

the soft layer satisfies the following relational expressions 1 and 2, $[C_{t / 5}] / [C_{M}] < 0.3$ $[C_{3 t / 5}] / [C_{M}] < 0.6$

(where, in the relational expression 1 and 2, [C_t/5] represents an average C content in a region from a surface to 1/5 of a predetermined depth (t) of the steel sheet in a thickness direction, [C_3t/5] represents an average C content in a region corresponding to 3/5 of the predetermined depth (t) of the steel sheet in the thickness direction from 1/5 of the predetermined depth (t), and [C_M] represents an average C content of the steel sheet).
The cold-rolled steel sheet of claim 1, wherein a fraction of the tempered martensite is 40% or more.
The cold-rolled steel sheet of claim 1, wherein the predetermined depth (t) is 50 to 100µm.
The cold-rolled steel sheet of claim 1, wherein the cold-rolled steel sheet has at least one of a hot-dip galvanized layer (GI), an alloyed hot-dip galvanized layer (GA), and an electrolytic galvanized layer (EG) formed on at least one surface thereof.
The cold-rolled steel sheet of claim 1, wherein the cold-rolled steel sheet has a 30000MPa% ≤ X = yield strength × [total elongation + (2 × hole expansion ratio)] ≤ 70000MPa%.
A method for manufacturing a cold-rolled steel sheet, comprising:
heating a slab at 1100 to 1300°C, the slab including, by wt%, carbon (C): 0.15 to 0.25%, manganese (Mn): 1.5 to 2.5%, silicon (Si): 1.0 to 2.0%, phosphorus (P): 0.1% or less (excluding 0%), sulfur (S): 0.03% or less (excluding 0%), aluminum (Al): 0.01 to 0.1%, molybdenum (Mo): 0.01% or less (excluding 0%), boron (B): 0.001% or less (excluding 0%), and a balance of Fe and other inevitable impurities;

finishing hot-rolling the heated slab at Ar3 or higher to obtain a hot-rolled steel sheet;

coiling the hot-rolled steel sheet at 700°C or less;

pickling and then cold-rolling the coiled hot-rolled steel sheet to obtain a cold-rolled steel sheet;

primarily heating the cold-rolled steel sheet at 780°C or higher and less than Ac3-10°C for 30 seconds or longer under an atmosphere condition having a dew point temperature of 0 to 30°C;

primarily cooling the heated cold-rolled steel sheet to 600 to 750°C at an average cooling rate of 1 to 10°C/s;

secondarily cooling the primarily-cooled cold-rolled steel sheet to 150°C~Ms at an average cooling rate of 10 to 45°C/s; and

secondarily heating the secondarily-cooled cold-rolled steel sheet at Ms~480°C and then subjecting the steel sheet to an overaging treatment for 1 to 30 minutes.
The method for manufacturing a cold-rolled steel sheet of claim 6, wherein the cold rolling is performed at a cold reduction ratio of 30 to 80%.
The method for manufacturing a cold-rolled steel sheet of claim 6, further comprising:
immersing the cold-rolled steel sheet in a hot-dip galvanized bath at 440 to 480°C after the secondary heating and holding, to form a hot-dip galvanized layer.
The method for manufacturing a cold-rolled steel sheet of claim 8, further comprising:
performing an alloying heat treatment on the cold-rolled steel sheet at 450 to 520°C after the formation of the hot-dip galvanized layer.
The method for manufacturing a cold-rolled steel sheet of claim 6, further comprising:
forming an electro-galvanized layer after the secondary heating and holding.